Some thoughts:
- "Virtual surround" is no longer tied to specific hardware solutions - Razer Surround Pro and Creative SBX3 are available as stand-alone software packages that will bring their respective headphone surround simulacra to any HDA-compliant audio device (this means basically everything). Alternatively, many games have built-in "headphone mode" which applies some degree of virtual surround (this varies heavily from game to game). There are also plenty of USB-based solutions from a wide range of manufacturers (e.g. Creative, Asus, Turtle Beach, Astro Gaming, etc) that offer various headphone surround simulacra in their drivers (e.g. DTS:X Headphone, Dolby Headphone, Creative SBX, etc), in addition to many surround sound receivers featuring Dolby Headphone functionality these days (this is a somewhat unwieldly option, but sometimes surround sound receivers can be had fairly cheap).
- Generally cheap PC speakers are limited mostly by being cheap PC speakers - soundcards for PCs have been largely "good enough" quality-wise since the early 2000s (when "they" started getting serious about multimedia, home theater, and eventually hi-fi), and modern cards have minimal disctinction between "mid range" and "high end" in terms of quality as either digital transports or analog line-out sources - usually the more premium cards (like say, Sound Blaster ZxR) distinguish themselves by offering built-in headphone amplifiers, more inputs/outputs, or bundling other "stuff" in the package (like a microphone). What I mean is, if you're just going to use this card as a digital S/PDIF source, or even as a stereo analog source, there's not a lot to be said for something like ZxR over the "base" Z model, and the same goes for the Asus Xonar and HT Omega cards too.
Overall I'd say "sound cards" have taken an awful rap over the last ten or so years, especially as hi-fi marketers have needed new and interesting ways to sell products to consumers new and old, and the "USB DAC" was born as a result of that (the short version: "USB DAC" is an inaccurate description - all of those USB devices are sound cards, but many of them don't offer much in the way of inputs or outputs, and some of them rely on generic drivers to further simplify things (which saves their manufacturers a ton of money on development and support)). Generally speaking, there's not a "significant good reason" to argue one way or the other here unless A) you're dealing with a mobile device wherein an internal PCI/PCIe card is out of the question or B) you're getting into nosebleed high end pricing (at which point we can start having estoeric arguments like: "can you *really* hear the difference and is that difference *really* worth the extra $10k?"). But as far as inexpensive USB audio devices vs PCIe or PCI cards, all of the above can offer great performance.
This isn't to say "sound cards" are beyond reproach - they have their own share of considerations, like expansion slot availability, reliance on (generally speaking) proprietary drivers, funkiness with said drivers, etc. This has all improved in leaps and bounds over the last 20 years, just as USB audio has also improved ("everything is better in the future"), but certainly there are understandable reasons where a basic USB audio device that relies on generic drivers may be preferable. My point is that we shouldn't generalize so aggressively one way or another, mostly because it comes down to arguing about the host interface as some sort of metric that can divine "fidelity" - its like debating if AGP or PCI Express will produce "higher quality graphics."
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I would heavily disagree with these assertions, mostly on logical and factual grounds. Some reasons why:
- The internal soundcard is also drawing power "via a DC source" and there's actually nothing that dictates said "DC source" for the external device will be less noisy (as some sort of generalization) than what's going on inside the PC. In most cases the cheap USB audio devices are actually drawing from the PC's internal power source, so you're dealing with the same PSU at the end of the day, but that 5VDC supply is coming through the USB circuitry, vs what's delivered to the PCIe (or PCI) slots. Modern computer PSUs tend to have very clean outputs (especially if you've got a high performance model from a fancy manufacturer), and modern motherboards tend to have good power sections on them too (especially if you've got a high performance model from a fancy manufacturer), so this is basically a moot point. If you're getting into an "externally powered" device (e.g. it comes with a wall-pack or plugs directly into the wall) now you're paying more (to provide that power source) and you're hoping that the manufacturer is including something that has better regulation, lower ripple, etc ("how much does this really matter in audible terms?" - it depends) than your PC's PSU. Usually wall-warts are going to be worse than what's in the PC, higher end devices with dedicated internal PSUs (and usually relatively large footprints and so forth) may swing this the other way, but you're still connecting to the PC electrically, so if there's lots of nastiness that doesn't magically go away because "its external" (unless of course, you're an advertiser).
- There is no "processing" in modern PC audio. It's all software based. This has been the way of things since Windows Vista. So you aren't "offloading" or "ditching" anything. You're just using USB in place of PCI or PCIe as a host interface to connect an audio controller/codec to the PC. That device, in turn, may have a DAC built-in (many USB audio controllers do, as they're designed as low-cost commodity products), or may interface to a separate DAC chip. This exists the same for PCI and PCIe cards too. Technically the internal cards benefit from lower latency and higher bandwidth connections to the system, but for basic playback at normal datarates (which includes gaming) that stuff generally isn't of significance (it does start to matter for music production though). So while you are moving the DAC out to whatever external interface, vs having it on an internal card, the merits of such a thing are, to put it lightly, debateable.
There are plenty of internal cards that measure more or less "flawlessly" (and I can cite this back over a decade if you're really interested), and in many cases they can post better measurements than many cheap USB devices. That doesn't mean you should generalize the other way, that "USB bad, internal good" - the point is that you shouldn't make such a generalization either way, because its not based on the whole picture.
Amplification (especially for hard to drive cans or speakers) is where things get more interesting. For USB bus powered devices, you're limited to a relatively modest 2.5W DC of power for a single port (500 mA @ 5V), whereas if the device brings its own power (so it only uses USB for data, and plugs into the wall on its own), it can draw "unlimited" power - so doing even a 100Wpc amplifier section isn't as much of a big deal, nor is needing higher supply voltages, or whatever else. This is one area where external devices have consistently been "ahead," and yet (in true consumer electronics fashion) probably the least talked about by marketers. Internal soundcards with built-in amps (which rely on the recent-ish generation of headphone-oriented chip amps from major manufacturers like TI or Maxim) have closed this gap somewhat, but even with the higher power available to PCI and PCIe devices, they will never fully compete with an external amplifier on a level playing field. For most headphones, however, the built-in amps are more than enough for regular usage (e.g. the Sound Blaster Z can put put around 250 mW/ch into 32 ohms - its not a ton, but its at least an order of magnitude more than most headphones need to get "quite loud").
As far as what should you buy - it really comes down to what you actually want, an external box, or an internal card. Either way, at around $50-100 there are plenty of good options. Creative has some USB devices that include a microphone and headphone amp, which could simplify life a bit in terms of connectivity, or you could go for an internal card from Asus for more like $50, and either rely on the fairly modest headphone amp available (the headphones you pick will determine whether or not this will be effective) or feed it into an external amp. Personally I'm a fan of "stand alone" headphone amplification (or amplification in general) so that you aren't chained to a source component just to have amplification for your headphones/speakers, so you can get whatever is the "best" source device and all it has to worry about is providing analog line-out at the end of the chain, and then the amplifier is an amplifier. But, that may be more "clutter" than your living space allows (and I certainly get that too), so all-in-one devices can be a godsend there.
Some cards to look at:
- Sound Blaster Z
- Xonar DGX and DSX
Some USB devices to look at:
- Sound Blaster Omni3D
- Xonar U7/U5
I haven't personally auditioned all of the above, but functionally any/all would satisfy your needs. Another thing to consider, if you are coming from a desktop PC (especially if you have PCI slots avialable) - internal soundcards generally don't hold their value very well, so you can usually find some pretty nice boards on the used market if you don't mind them being a few years old, like the X-Fi cards, the Asus Xonars, the Razer AC-1, among various others. Be cognizant of driver support if you go digging though (especially with PCI cards - some of this stuff may date back to the late 90s or early 2000s and have no modern driver support), and I'd also (unfortunately) say be leery of the Auzentech cards - they have awesome hardware, but the company seems to have vanished, along with their website and driver downloads.
tl;dr: Generalizing USB vs "internal" is kind of fruitless - get a device that has the connectivity you need, don't fear (or be wowed by) lack of (or inclusion of) headphone surround as that can all be done by stand-alone software packages, and expect that good performance can be had from either internal or external solutions.
EDIT
This isn't the headphone section, but I wanted to comment on your cans for gaming too: the HD 580/6x aren't bad headphones at all (in fact, they're really very good), but if gaming is the primary focus, I'd probably save the bucks and go for the HD 59x or 55x which tend to have (from my recollection) a bit wider soundstage, and be a bit lighter - you sacrifice some detail and "absolute fidelity" but for gaming that's largely a moot point. I don't know what the Massdrop 6XX cost though, so if they're available at a killer price (like down there with the 558 or 595) that would be a serious contender too.
If not Sennheiser, the AKG 700-series are probably where I'd look next. IMHO they're better in terms of positional audio and "3D-ness" (what? its sort of a word...) but you give up some bass and some folks have an issue with their soundstage (specifically that it feels a bit artificially big). Either would be a great consideration. Beyerdynamic DT880s round out the "old school triple crown" and are a decent middle-ground in terms of "3D-nes" and "natural-sounding-ness" but they're also on the lighter side, presentation wise, and may come across as piercing/bright to some listeners. But all three deserve at least a passing look, and they'd all deliver excellent performance both for music and gaming (and you'd really be at a point where "upgrading" is into the territory of diminishing returns - don't let the modern adverts sell you on these being "entry level" cans - they're bona fide audiophile quality, they're just ten to twenty years old, and you can get another 1-10% more performance with some other offerings (in some cases at 10-100x the price)).
